I was considering buying a five speed knob and just replacing the five speed insert with the six speed insert. Dealers still seem to have the five speed version in stock for the dealer price. I’m not sure if Sopers in Lincoln have a five speed knob and a six speed insert, in which case, you’d be in business for a 10% reduction and free p+p for mentioning the z4 forum. Ocean Plymouth are selling the five speed version on fleabay, so must have them in stock. Here are a few links and part numbers I found:
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F172982035951
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F202354203706
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F391913110552